Iron Uptake in Host Defense Against Pneumonia

Iron acquisition is a crucial component in microbial pathogenesis and determines the outcome of many diseases. This thesis examines the role of iron uptake in two distinct models of pneumonia caused by bacterial and fungal with the ultimate goal of identifying novel therapies. Gram-negative pneumonia is a dangerous illness which is difficult to treat due to antibiotic resistance among the causative pathogens. Hepcidin is the master regulator of extracellular iron availability in vertebrates, but its role in the context of host defense is undefined. We hypothesized that hepcidin-mediated depletion of extracellular iron during Gram-negative pneumonia protects the host by limiting dissemination of bacteria to the bloodstream. During experimental pneumonia, hepcidin was induced and mediated a rapid decline in plasma iron. In contrast, hepcidin-deficient mice developed a paradoxical increase in plasma iron during infection which was associated with profound susceptibility to bacteremia. Incubation of bacteria with iron-supplemented plasma enhanced bacterial growth in vitro, and systemic administration of iron to WT mice similarly promoted increased susceptibility to bloodstream infection. Finally, treatment with a hepcidin analogue restored hypoferremia in hepcidin-deficient hosts, mediated bacterial control, and improved outcomes. These data show hepcidin induction during pneumonia to be essential to preventing bacterial dissemination by limiting extracellular iron availability. Hepcidin agonists may represent an effective therapy for Gram-negative infections in patients with impaired hepcidin production or signaling.
Invasive pulmonary aspergillosis is a devastating fungal pneumonia which affects severely immunocompromised patients. Mortality remains high even with aggressive antifungal therapy, highlighting the need for more therapeutic options. It has been shown that high-affinity iron scavenging is crucial for the pathogenesis of invasive pulmonary aspergillosis, and that heme-containing proteins are necessary for Aspergillus hyphal growth and resistance to oxidative stress. The most abundant source of iron in the body is heme, but there are conflicting reports in the literature as to whether Aspergillus species are able to utilize heme. We hypothesized that heme uptake is important in the pathogenesis of invasive pulmonary aspergillosis. We found that, during the first 3 days of experimental invasive aspergillosis, both total and extracellular lung heme iron content increased as the infection progressed due to pulmonary hemorrhage and angioinvasion. We found that treatment with exogenous hemin, but not tin protoporphyrin, worsened the outcome of infection without appearing to promote fungal growth. In minimal media, exogenous hemin promoted A. fumigatus growth, conidiation, and resistance to oxidative stress in vitro. Tin protoporphyrin accumulated in the cell wall and cytosol of A. fumigatus hyphae. Together, these data suggest that A. fumigatus can utilize exogenous heme under certain circumstances and that heme uptake should be further explored as a virulence mechanism.